The proposed studies will examine sex differences in Attention Deficit Hyperactivity Disorder (ADHD) during the transition from late childhood to adolescence. This research will determine: (1) whether the factor- structure of DSM-IV ADHD symptoms differs for males or females during the transition from late childhood through adolescence, (2) whether DSM-IV diagnostic criteria for ADHD under-identify females who experience ADHD related impairment, and (3) whether males and females follow similar developmental trajectories of ADHD symptomatology during the transition from late childhood through adolescence. Collectively these studies will facilitate efforts to refine the diagnostic criteria for ADHD under-identify females who experience ADHD related impairment, and (3) whether males and females follow similar developmental trajectories of ADHD symptomatology during the transition from late childhood through adolescence. Collectively these studies will facilitate efforts to refine the diagnostic criteria for ADHD. Important aspects of this proposal include: (1) Secondary data analysis of a high quality, longitudinal, epidemiological data set, (2) Assessing ADHD using DSM-IV criteria that distinguish between inattentive and hyperactive-impulsive symptom types, (3) Approaching the question of sex differences in ADHD from an explicitly developmental perspective, and (4) Utilizing advanced quantitative methods that address the proposed research questions that do traditional methods.